Debug weird UI behavior with CDP logging

Testing & Quality

Use when weird app/UI behavior needs live reproduction and tracing, including flicker, stale state, remounts, focus/scroll jumps, action glitches, or ordering flashes

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Debug weird UI behavior with CDP logging

Use this when the app does something weird and the behavior must be observed live: refreshes, remounts, flickers, rows briefly unfolding, wrong ordering for a moment, state changing then reverting, actions firing twice, stale UI, unexpected navigation, focus loss, scroll jumps, or anything where code inspection alone is likely to lie.

Rules

Reproduce in the running dev app. Do not guess from code only.
Use CDP against Electron (CDP_PORT=39217) when possible.
Instrument the page with a small in-page logger before performing the action.
Only act on disposable data.
- Create temp branches/worktrees/projects/sessions when needed.
- Use obvious names like cdp-debug-* or tmp-*.
- Delete/clean them up after the test.
- Do not use important branches, real user sessions, or serious project data for destructive actions.
If a destructive action is required, make the temporary object first, verify it appears, then delete that object only.

CDP setup

List targets:

CDP_PORT=39217 /home/igorw/.pi/agent/skills/chrome-cdp/scripts/cdp.mjs list

Use the target id prefix from list in later commands.

Install a DOM mutation logger

Adapt selectors to the surface being debugged. For sidebar branch/worktree flicker, this logger captures row labels, expanded state, and scroll position:

CDP_PORT=39217 /home/igorw/.pi/agent/skills/chrome-cdp/scripts/cdp.mjs eval <target> '(() => {
  window.__uiDebug?.observer?.disconnect?.()
  const root = document.querySelector(".sidebar-project-work-section")
  window.__uiDebug = { events: [], startedAt: Date.now() }
  const labels = () => Array.from(document.querySelectorAll(".sidebar-project-work-branch-toggle"))
    .map((el) => ({
      text: el.textContent.trim(),
      expanded: el.getAttribute("aria-expanded"),
      disabled: el.hasAttribute("disabled"),
    }))
  const snap = (reason) => window.__uiDebug.events.push({
    t: Date.now(),
    dt: Date.now() - window.__uiDebug.startedAt,
    reason,
    labels: labels(),
    scrollTop: document.querySelector(".sidebar-project-work-scroll-shell")?.scrollTop ?? null,
  })
  snap("install")
  const observer = new MutationObserver((mutations) => {
    if (!mutations.some((m) => m.type === "childList" || m.type === "attributes")) return
    snap("mutation:" + mutations.map((m) =>
      m.type + (m.attributeName ? ":" + m.attributeName : "") + ":+" + m.addedNodes.length + ":-" + m.removedNodes.length
    ).slice(0, 8).join(","))
  })
  if (root) observer.observe(root, {
    subtree: true,
    childList: true,
    attributes: true,
    attributeFilter: ["aria-expanded", "data-current", "data-divider-after", "data-divider-before"],
  })
  window.__uiDebug.observer = observer
  return labels().slice(0, 20)
})()'

Reproduce with temp data

Example pattern:

Create a temp thing (cdp-debug-*).
Confirm it appears.
Add a manual marker before the suspect action:

CDP_PORT=39217 /home/igorw/.pi/agent/skills/chrome-cdp/scripts/cdp.mjs eval <target> '(() => {
  window.__uiDebug.events.push({ reason: "before-action", t: Date.now(), dt: Date.now() - window.__uiDebug.startedAt })
  return true
})()'

Perform the suspect action.
Dump recent events:

CDP_PORT=39217 /home/igorw/.pi/agent/skills/chrome-cdp/scripts/cdp.mjs eval <target> '(() => window.__uiDebug?.events?.slice(-80))()'

What to look for

Full page reload:
- CDP state disappears, window.__uiDebug is gone, or Vite logs full reload.
- Check dev server watcher config, especially generated folders/worktrees.
Real component/state flicker:
- Logger persists and shows multiple DOM mutations.
- Compare snapshots before/after each mutation.
- Look for stale mixed sources of truth: cache patched before related query/state refreshed, or vice versa.
Remount/state reset:
- Local UI state resets while no page reload happened.
- Check keys, conditional rendering, parent state sync effects, and broad query writes.

Fix strategy

Prefer one consistent state transition over optimistic patch + stale secondary state.
If UI derives from two sources, update them in an order that avoids impossible intermediate states.
Preserve object references on broad cache merges when data is unchanged.
Avoid full-shell/query refreshes for localized mutations.
Add a regression test for the specific ordering/reference behavior when practical.

Cleanup

Remove temp branches/worktrees/projects/sessions created for the test.
Re-run the logger after the fix with a fresh temp object.
Keep unrelated local changes unstaged.
Run the repo verification command when code changed.